Manganese and its compounds

The electron configuration of manganese

[Ar]4s²3d⁵

Properties of manganese as a transition element

1. Forms colored compounds; for instance, Mn⁷⁺ is purple
2. Has variable oxidation states; +2, +3, +4, +5, +6, +7
3. Forms complexes, e.g. MnO₄^–.
4. Has catalytic properties, e.g., MnO₂ catalyzes the release of oxygen from hydrogen peroxide

The reaction of manganese with air

Manganese is not very reactive in regards to air. The surface of manganese lumps oxidizes a little. Finely divided manganese metal burns in air. In oxygen, the oxide Mn₃O₄ is formed and in nitrogen, the nitride Mn₃N₂ is formed.

3 Mn(s) + 2 O₂(g)  →       Mn₃O₄(s)
3 Mn(s) + N₂(g)     →       Mn₃N₂(s)

The reaction of manganese with Halogens

Manganese reacts with the halogens, forming the corresponding manganese(II) halides. For fluoride, manganese(III)fluoride is also formed

Mn(s) + F₂(g)   → MnF₂(s)
2 Mn(s) + 3 F₂(g)   → 2 MnF₃(s)
Mn(s) + Cl₂(g)   → MnCl₂(s) [3]
Mn(s) + Br₂(g)  →  MnBr₂(s)
Mn(s) + I₂(g)  →  MnI₂(s)

The reaction of manganese with water

Manganese reacts slowly with water forming manganese dioxide

Mn(s) +2H₂O(l) → MnO₂ (s) + 2H₂(g)

The reaction of manganese with acids

Manganese dissolves readily in dilute sulphuric acid, forming a colorless solution of Mn(II) ions and hydrogen gas, H₂.

Mn(s) + H₂SO₄(aq)  →  Mn²⁺(aq) + SO₄²⁻(aq) + H₂(g)

Important compounds of manganese

Potassium Manganate (VII), KMnO₄

– It used as an oxidizing agent to oxidize alcohols and aldehydes to carbonyl compounds and carboxylic acids respectively.

– it is used to standardize iron (II), Hydrogen peroxide  and  oxalic acid

Half equation for manganate 7

MnO₄^– (aq) + 5e + 8H⁺(aq) → Mn²⁺(aq) + 4H₂O(l)

Trial 1

Complete and balance the following equations

Fe²⁺ (aq) + H⁺ (aq) + MnO₄^–(aq) →

H₂O₂ (aq) + H⁺ (aq) + MnO₄^–(aq) →

  C₂O₄^2-  (aq) + H⁺ (aq) + MnO₄^–(aq) →

Manganese (VII) oxidizes halides in the preparations of halogens

Trial 2

Complete and balance the following equations

Cl^– (aq) + H⁺ (aq) + MnO₄^–(aq) →

For this reason HCl is not used to acidify MnO₄^– in its oxidation reaction because Cl^– ion is oxidized as well.

Variation of oxidizing power of manganate (VII) ions with pH.

The oxidizing power of potassium permanganate depends on the pH as shown below:

(a) Acidic solution

MnO₄^– (aq) + 8H⁺ (aq) + 5e →Mn²⁺ (aq) + 4H₂O (l)

(b) Neutral or alkaline solutions,

The manganate (VII) is reduced to manganese (IV) oxide.

MnO₄^– (aq) + 2H₂O (l) + 3e → MnO₂ (aq) + 4OH- (l)

(c)Strongly alkaline solution

Manganate (VII) is reduced to green manganate (VI).

MnO₄^– (aq) + e-  → MnO₄^2- (aq)

Disadvantages of using potassium permanganate (VII) in volumetric analysis

• Manganate (VII) cannot be used as a primary standard because

(i) it cannot be obtained in pure form. The impurities are MnO₂.

(ii) Aqueous solutions of manganese (VII) is unstable and slowly deposit brown manganese (IV) oxide on standing.

(b) Hydrochloric acid cannot be used as an oxidizing agent of potassium permanganate VII

(c) Nitric acid cannot be used to oxidize potassium permanganate (VII) solution because it is an oxidizing agent

Trial 3

(a) Potassium manganate (VII) is not used as a primary standard in volumetric analysis and has to be standardized

(i) What is a primary standard?  (2marks)

(ii) State three characteristics of a primary standard.                                (3 marks)

(iii) Explain why potassium manganate (VII) is not used as a primary standard.      (2 marks)

Name one substance, other than ethane dioc acid which can be used as a primary standard for potassium manganate.  (VII)        (1 mark

(b)  Explain why hydrochloric acid is not used usually to acidify solutions of potassium manganate (VII)                                                                    (2 marks)

(c) Acidified potassium manganate (VII) reacts with ethane dioc acid

Write

• The half equations for the reaction.         (2 marks)
•  The overall equation for the reaction.    (2 marks)

(d) 20.00 cm³ of 0.01 M manganate (VII) ions solution required exactly 16.65 cm³ of a solution containing 5.1 g l^-1 of an oxalate (COO^–X⁺)₂.2H₂O. Calculate the atomic mass of X.

Trial 4 

Iron (II) sulphate is normally used to standardize a solution of potassium manganate (VII) acidified with sulphuric acid

(a) Write an equation for the reaction between potassium manganate and iron (II) sulphate.(1 ½ marks)

(ii) State why hydrochloric acid is not used to acidify potassium manganate (VII).  (2 ½ marks)

(b) 20.00 cm³ of acidified solution of 0.02M potassium manganate (VII) reacted exactly with 25.10cm³ of sodium nitrite. Potassium manganate (VII) reacts with sodium nitrite according to the following equation.

2MnO₄^– (aq) + 5NO₂^–(aq) +6H⁺(aq) → 2Mn²⁺(aq) + 5NO₃^–(aq) +3H₂O(l)

Calculate the concentration of the sodium nitrite in moles per litre.

Manganese (VI) compounds:

The sodium and potassium manganates (VI) are the only pure compounds containing manganese in the +6 oxidation state. Both these compounds are dark green solids, the colour being due to the MnO₄^2- ion.

Manganese (VI) disproportionate in acidic medium to give purple manganate VII ions

3MnO₄^2- (aq) + 2H₂O (l) → 2MnO₄^– (aq) + MnO₂ (s) + 4OH^– (aq)

Definition

A disproportionation reaction is a reaction where there is concurrent oxidation and reduction

Disproportion of manganese (VI) is speeded up by bubbling a stream of carbon dioxide through the solution, since this removes the hydroxyl ions as hydrogen carbonate ions

 4^–OH (aq) + 4CO₂ (g)  → 4HCO₃^– (aq)

Manganese (IV) compounds

Manganese (IV) oxide, MnO₂, is the main manganese (IV) compound; it occurs naturally as the ore pyrolusite; and it can be made by the action of heat on manganese (II) nitrate.

 Mn(NO₃)₂ (s)  →   MnO₂ (s) + 2NO₂ (g)

Uses of manganese IV oxide

(i) preparation of chlorine

MnO₂ (s) + 4HCl (aq) → MnCl₂ (aq) + 2H₂O (l) + 2Cl₂ (g) 

(ii) it is a catalyst in the preparation of oxygen from hydrogen peroxide.

The reaction of Manganese (IV) compounds.

In the presence of air or oxidizing agent (e.g. KNO₃, KClO₃) MnO₂ reacts with fused KOH to give potassium manganate, K₂MnO₄.

2MnO₂(s) + 4KOH(l) + O₂ (g)→ 2K₂MnO₄(aq) + 2H₂O(l)

Qualitative analysis of  Manganese II

(a) Addition of dilute sodium hydroxide or ammonia solution dropwise until in excess.

A white ppt. insoluble in excess, rapidly turning brown.

Mn²⁺ (aq) + 2^–OH (aq)  → Mn(OH)₂ (s)

Then

4Mn(OH)₂ (s)  + O₂(g) →    Mn₂O₃(s) + 2H₂O(l)

Confirmatory test of Mn²⁺

Mn²⁺ aqueous solution turn purple solution on addition concentrated nitric acid followed by sodium bismuthate or Lead (IV) oxide.

2Mn²⁺ (aq) + 5BiO₃^– (s) + 14H⁺ (aq) → 2MnO₄^– (aq) + 5Bi³⁺ (aq) + 7H₂O (l)

2Mn²⁺ (aq) + 5PbO₂(s) + 4H⁺ (aq) → 2MnO₄^– (aq) + 5Pb²⁺ (aq) + 2H₂O (l)

Trial 5 

A crystalline solid Y dissolved in water to give a pink solution. The addition of excess aqueous sodium hydroxide produced a dirty white precipitate which rapidly turned brown on standing.

When nitric acid was added to the solution Y followed by sodium bismuthate solution, the solution changed from pink to purple.

(a) Identify the cation in Y. (1 mark)

(b) Write:

(i) The equation for the reaction that took place when sodium hydroxide was added to the solution. (1½ marks)

(ii) The formula of the species responsible for the purple color.       (1 mark)

(iii) The equation for the reaction leading to the formation of the brown solid. (1½ marks)

Trial 6 

• Write the outer electronic configuration of manganese.  (1 mark)

(i) State the possible oxidation states of manganese.                               (2 ½ marks)

(ii) How does the acidity of the oxides of manganese vary with increasing oxidation states?                                                                                   (½ marks)

(iii) What is the change in the oxidation state of manganese when potassium manganate (VII) is reduced in alkaline medium?                             (1 mark)

(iv) State four applications of potassium manganate (VII) in the laboratory as oxidizing agent?                                                                                    (4 marks)

Suggested answers

Trial 1

5Fe²⁺ (aq) + 8H⁺ (aq) + MnO₄^–(aq) → 5Fe³⁺(aq) + Mn²⁺(aq) + 4H₂O(l)

5H₂O₂ (aq) + 6H⁺ (aq) + 2MnO₄^–(aq) → 2Mn²⁺(aq) + 8H₂O(l)

  5C₂O₄^2-  (aq) + 16H⁺ (aq) + 2MnO₄^–(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)

Trial 2

10Cl^– (aq) + 16H⁺ (aq) + 2MnO₄^–(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 5Cl^–(aq)

Trial 3

• (i) A primary standard is a substance of known high purity which may be dissolved in a known volume of solvent to give a primary standard solution

(ii) Features of a primary standard include:

• High purity.
• Stability (low reactivity)
• Low hygroscopicity (to minimize weight changes due to humidity)
• High equivalent weight (to minimize weighing errors)
• Non-toxicity.
• Ready and cheap availability.
• Chloride ions react with manganate (VII) ions.

2MnO₄^– (aq) + 10Cl^– (aq)+ 16H⁺ (aq)  → Mn²⁺(aq) + 5Cl₅(g) + 8H₂O(l)

• (i) MnO₄^– (aq) + 8H⁺ (aq) + 5e →Mn²⁺ (aq) + 4H₂O (l)

    C₂O₄^2- (aq) -2e → 2CO₂(g)  

(ii) 5C₂O₄^2-  (aq) + 16H⁺ (aq) + 2MnO₄^–(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g)

• X =23

Trial 4

0.04M

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